Field Effect Transistors (FETs) are well known to be ideal for use in applications requiring amplification or switching at radio or microwave frequencies. FETs fabricated primarily of GaAs are particularly suited for high frequency uses because of the high electron mobility characteristic of this compound semiconductor. In the past, FETs have utilized a Schottky barrier gate structure (hence the common name Metal-semiconductor Field Effect Transistor, or MESFET) and have been fabricated on semi-insulating GaAs substrates with all dopants being ion implanted.
Recently, the performance demands of modern radar and telecommunications equipment have outstripped the capabilities of traditional MESFET technology. Consequently, FETs have evolved into largely epitaxial structures where semiconductor layers are precisely grown and doped in situ in the growth process. One new type of FET is the High Electron Mobility Transistor (HEMT). A prime difference between the HEMT and FET is in where the source-drain current flows. In the FET, the electrons flow in a doped channel layer, through the donor ions, and therefore undergo considerable scattering. In the HEMT, however, a potential well is created at a heterojunction interface (typically between GaAs and AlGaAs) due to the different conduction band energies for the two materials. The electrons come from the doped material having the larger energy bandgap (AlGaAs) and reside in the potential well, forming a two-dimensional electron gas. Since the electrons travel in the undoped smaller bandgap material (GaAs), and are physically separated from the donor ions, ionized impurity scattering is greatly reduced and the electrons move with higher mobility and velocity. This results in a device with superior microwave and radio frequency performance characteristics.
However, for frequencies above 60 GHz, conventional HEMTs have been shown to perform poorly because of low gain. The reason for this is thought to be that the electrons in the two-dimensional electron gas are difficult to confine at the short gate lengths required for this high-frequency operation. Consequently, a new type of HEMT has emerged that possesses improved carrier confinement. This device is known as a PHEMT, for Pseudomorphic High electron Mobility Transistor. The PHEMT differs from the conventional HEMT in that the channel layer is comprised of an even narrower bandgap material than in the HEMT. This channel is typically a thin (100-200 Angstroms) layer of InGaAs. The device is therefore based on the InGaAs/AlGaAs heterojunction, with electrons flowing in the undoped InGaAs. The benefits of using InGaAs as the channel include the enhanced electron transport in InGaAs as compared to GaAs, the large conduction band discontinuity which allows high 2DEG density and hence high current, and improved confinement of carriers due to the quantum well. PHEMTs have been shown be useful for frequencies as high as 100 GHz.
A further refinement of the PHEMT is the inverted PHEMT. The inverted PHEMT is characterized by having its channel layer vertically closer to the gate than the AlGaAs ion donor layer, which is in contrast to the conventional PHEMT. Inverted PHEMTs have exhibited very good low noise performance at low DC powers, a result thought to be attributable to the close gate and channel proximity and superior carrier confinement.
Another type of FET has evolved to permit operation with higher breakdown voltages and therefore at higher power than was possible with traditional MESFETs, and even HEMT devices. One method to achieve higher breakdown voltages in the past has been to incorporate an AlGaAs buffer layer atop a GaAs channel layer. The AlGaAs layer is undoped or lightly doped and separates the highly doped GaAs channel from the gate contact placed on top of the AlGaAs layer. This device is known generally as a MISFET (Metal Insulator Field Effect Transistor), or heterostructure FET (HFET), because of the "insulating" AlGaAs layer. Furthermore, HFETs have been shown to be very efficient devices for power applications. Power added efficiencies (PAEs) of over 75% have been reported with these devices.
Radar and telecommunications systems commonly require a low-noise device for receiver amplifiers, while also requiring high power and robust devices for power amplifiers used in transmitting applications. This has traditionally required a system designer to have integrated circuits for power amplifiers, integrated circuits for low-noise amplifiers, and even integrated circuits for the switching and phase shifting functions commonly used in these systems.